In many processes, the resulting exhaust gas is cleansed by an electrostatic precipitator which employs a high voltage to ionize and deflect particulate matter. Known precipitator systems control the magnitude of this high voltage by adjusting it to produce sparking at a predetermined rate. Since sparks are easily detected, such spark-rate control is readily implemented but only with a great loss in performance and reliability. By allowing repeated sparking, these systems severely stress the power supply and its transformer-rectifier. In addition, the collecting plates and emitting wires of the precipitator are continually eroded.
Known precipitator systems have responded to excessive sparking or arcing that randomly occurs during typical operations, by quickly disabling the high voltage supply and then restoring it gradually. This gradual restoration known as a "slow" or "soft" start, avoids excessive initial currents and voltages. Such excessive initial loads are the consequence of a heavy spark or arc driving magnetic devices in the high voltage supply into saturation.
In the event that the core of a high voltage transformer saturates, reapplication of power without regard to voltage polarity can result in further saturation and a consequently low impedance path that shunts excessive current from the power mains. Alternatively, a conventional series reactor in the primary circuit of the high voltage transformer may saturate. In a similar fashion, this saturated reactor can provide insufficient limiting impedance so that the transformer is overdriven, producing at its secondary, excessive high voltage. The foregoing produces a self-defeating sequence whereby a spark produces a high voltage transient that causes another spark. The aforementioned "slow" start systems cope with this phenomena by gradually restoring drive to the magnetic circuits thereby allowing rebalance of their magnetic cores and operation on a central and symmetrically hysterisis loop.
An inherent disadvantage with delaying restoration of full power to a high voltage supply is that the charge within the precipitator decays. It is to be appreciated that while a spark constitutes a significant loss of energy, initially the effect is localized. Thus, adjacent precipitator sections can initially maintain their charges because the effective series resistance and inductance between precipitator sections is not negligible. In fact, these impedances become predominant when a spark produces a localized low impedance current path. Accordingly, sparks, which are commonly of a short duration (1 to 4 milliseconds), are followed by an interval in which charge can be advantageously redistributed. Thus a localized discharge can be immediately followed by a replenishment of charge and at a lower magnitude. However, since known system repress power beyond this redistribution interval, they allow the remaining overall charge to dissipate.
If the precipitator charge is allowed to decay in this fashion, the system is so disabled that collected dust can become reentrained. This reentrained dust increases the probability of another spark, again initiating a self-defeating sequence. Once the precipitator has been so discharged, the current handling limitations of the equipment, the capacitance of the precipitator and the inductance of the series reactor render it impossible to instantaneously restore the charge on the precipitator. Accordingly, these known systems devote a significant amount of time recharging unnecessarily discharged precipitators. This non-productive cycling defeats effective voltage control. The present invention by quickly restoring power, more nearly achieves the theoretical maximum power described hereinafter.
The deleterious effect of a ramp-type process can be illustrated by assuming that a spark is followed by an interval of duration DTa during which power is removed. Thereafter it is assumed that the charging current is linearly increased for an interval of duration DTb after which time a constant value of maximum current Im is reached. (The following analysis can be readily extended to the other forcing functions such as a piecewise linear ramp having an initially high and subsequently low slopes). Since the energy delivered is the effective resistance R multiplied by the time integral of the square of the current, total energy will be: ##EQU1## setting t=0 as the instant at which the charging current commences. If the foregoing is solved for t=Tf-DTa where Tf is the period at which the process repeats, the energy per cycle is: ##EQU2## Clearly then, the maximum value of energy occurs when DTa=DTb=0. Under those circumstances the maximum theoretical power is delivered which by inspection is: Im.sup.2 R. If the ramping interval corresponding to quanitity DTb were eliminated, the charging current would become a square wave and the power delivered then would be the maximum theoretical power multiplied by the duty cycle (DTa/Tf). This duty cycle can approach its maximum value of one, if the time between sparks is made arbitrarily large or the interval after sparking when power is repressed is made arbitrarily small. By alternately discharging and recharging the precipitator, these known systems effectively masked subtle electrical variations which are useful in controlling the high voltage. Because the transients caused by sparking and the inrush currents due to recharging are so large, these known control systems were designed to respond to an easily detectable phenomena: the spark itself. Moreover, while one might very gradually increase the drive to a high voltage supply in an attempt to detect subtle electrical variations, the net result is that the high voltage will languish at an inefficiently low voltage.
The present invention avoids these problems by operating in a stabilized fashion. During the occurrence of a spark, power is repressed to the extent possible, in one embodiment of the invention. Thereafter power is quickly restored to nearly the prespark value. In a preferred embodiment, power restoration is timed so that if sparking has caused saturation of a magnetic device of the high voltage supply, that device is immediately driven in a direction away from saturation.
In addition, in a preferred embodiment of the invention, the present value of the drive being applied to a high voltage converted is compared to a previous value. In this manner, electrical disturbances which are the precursors of sparking are detected. Accordingly, the system can adjust itself prior to the occurrence of a spark and thus avoid it. Therefore systems according to the present invention are designed to operate without sparking. Preferably, the adjustments made to avoid a spark are relatively small in magnitude so as not to disturb the essentially steady-state conditions achieved. Such quiescence facilitates continued detection of an imminent spark. With the foregoing technique the high voltage can be continuously adjusted upward to an efficient value without encouraging unnecessary sparking. The equipment produces increases of up to 200 percent in the real power available in the precipitator.
In a typical operation of the preferred embodiment, the conduction angle of anti-parallel silicon controlled rectifiers (SCRs) are increased at some rate that will provide a fast but controlled rise in the output of a high voltage transformer. The rate of increase of SCR conduction angle may be modified by feeding back to the control a signal derived from the voltage drop across a linear current limiting reactor included in the primary circuit, such signal being indicative of the rate of power acceptance of the precipitator itself. The control may also, for each individual half cycle of operation, respond to measured quantities such as the voltages and currents appearing in the transformer primary or secondary circuits (i.e., the precipitator). Such system can also include a means for storing those measured values for comparisons at the next or some later half cycle. In this way, when a spark occurs all of the latest values of all measurable electrical parameters are available for evaluation.
With the disclosed apparatus, a fast comparison can be made, for instance, between the secondary voltage and/or current at which the spark occurred and the SCR conduction angle required to produce it. Additional information could include: whether or not the SCR conduction angle was advanced or retarded at some time just prior to the spark, the SCR conduction angle itself, secondary voltage and/or current, linear reactor voltages or any other discernable characteristics existing prior to the spark. Thus the new "local time average" may be formed and the control would now be ready to resume operation. Since reenergization follows the method disclosed, the control may produce a new SCR conduction angle such that operation is resumed so as to immediately (within one or two half cycles) produce secondary voltages and/or currents at or just below those at which the first spark occurred. If another spark is generated, another local average can be obtained and the process repeated. In this way the secondary voltage and/or current is reduced until no spark occurs upon or shortly after reenergization. By decreasing the SCR conduction angle by small amounts (less than one electrical degree) it is possible to obtain long term (several dozen half cycles) operation just below the levels of voltage and/or current that previously produced a spark but more importantly, the system is allowed to stabilize quickly so that spark onset conditions may be readily detected.
If no evidence of spark onset is detected from monitoring half cycles, the SCR conduction angle may be increased but again only by a small amount and in a single step, so as to disturb the steady state conditions as little as possible. The control continues forming local averages and at some conduction angle, the spark onset disturbances will become detectable. At this point it is designers choice as to what type of action the control should take. Operation over several half cycles however, will indicate that continued operation at present levels of voltage and/or current will lead to a spark so the obvious action to take would be to decrease the SCR conduction angle by an amount which could depend on the amplitude (or distribution) of the onset disturbance. Data is then recollected to see in what way, if any, the disturbance was affected, and another decision made. Operation would be continued and the control would normally increase the SCR conduction angle whenever possible to provide maximum field potential and maximum ionizing current in a manner that is modified by the data being continuously collected, so as to rapidly lower the power by a very small amount when conditions indicate onset of sparking.
In a preferred embodiment the foregoing comparison measurement is performed periodically to eliminate periodic effects caused by an alternating energizing current.
Also, the instant at which a measurement is made can be set to correspond to the time at which current variations are most representative of the imminence of sparking. For example, sparking may be likely in a predetermined interval after a voltage controller is adjusted or, for alternating current, at predetermined phase angles.
In addition, embodiments incorporating principles of the present invention may repetitively increment a control signal which is controlling the extent to which a high voltage converter is driven. This process can be reversed if the high voltage begins to fall at the onset of back-corona.